1. Field of the Invention
The present invention relates to a light emitting device that has a light emitting element.
2. Description of the Related Art
Since a light emitting element has a self-light emitting property, it is superior in visibility and a viewing angle. Accordingly, a light emitting device that has a light emitting element has been attracted as well as a liquid crystal display device (LCD).
As a light emitting element, an organic EL element in which several organic layers are interposed between an anode and a cathode is given. The organic layer specifically includes a light emitting layer, a hole injecting layer, an electron injecting layer, a hole transporting layer, and an electron transporting layer. In such an organic EL element, light can be extracted by applying a potential difference between a pair of electrodes.
When a light emitting device is put into practical use, it is considered that life extension of the organic EL element is an important topic. Time degradation of the organic layer causes luminance reduction of the organic EL element. The rate of time degradation depends on material characteristics, a sealing method, a driving method of the light emitting device, or the like. In addition, since the organic layer is particularly weak in moisture, oxygen, light, and heat, the time degradation is also promoted by factors thereof.
In addition, when the light emitting device is put into practice use, the amount of current flowing in the organic EL element is desired to be constant without depending on the temperature. Even if the voltage applied between electrodes of the organic EL element is the same, the current flowing in the light emitting element becomes increase as the temperature of the organic layer becomes higher. In other words, when constant voltage driving is performed to the light emitting device, luminance change and chromaticity discrepancy occur in accordance with the temperature change. In the light emitting device having such an organic EL element, a technique in which the luminance of the light emitting element is to be constant independently from the environmental temperature is proposed (Patent Document 1: Japanese Published Patent Application No. 2002-333861).